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Divorce: Twin Studies of an Emergenic Trait

Presentation by: Christine Hood

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My Personal Interest in Twins

• Monozygotic twins: one is divorced and one is not.

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Usefulness of Twin Studies• Research: “Almost any kind of research that you

would think of doing with human subjects, psychological or medical, would be more interesting if you were doing it with twins.” –David Lykken

• Nature/Nurture Debate: Monozygotic twins can be thought of as “parallel forms of the same individual.” –Matt McGue

• Heritability: MZ-DZ

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Twin Studies on Divorce Heritability

• Lykken & Bouchard:-examined genetic influences-results: MZ =.45, 250% increase in risk

• McGue:-examined genetic and environmental influences-results: heritability =.42, slight indication of environmental influence

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What do twin studies and family studies show?

• A strong genetic influence, yet does not run in families.

• Somewhat influenced by the interaction from the environment.

• Studies suggest that risk for divorce has a complicated pathway of transmission, depending on the interaction of the genes and the environment.

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3 Ways of Genetic Transmission

1. Mendelian

2. Galtonian

3. Emergenesis

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Mendelian Transmission

• Mechanism in which the occurrence of a trait depends on the presence or absence of an allele at a particular locus.

• Is the focus of medical geneticists.

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Galtonian Transmission

• A mechanism of polygenic (many genes) transmission.

• Trait is determined by the additive action of a number of genes at different loci.

• Is the focus of behavior geneticists.

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Emergenesis

• A mechanism the involves a more general type of gene interaction.

• Also polygenic, yet the gene effects combine configurally rather than additively.

• This type of transmission is generally neglected by geneticists.

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A visible example of Emergenesis in the Human Body:

Voice• MZ twins have easily confusable voices; pitch and

resonance is remarkably similar.• Vocal determinants: shape and dimensions of chest and

head cavities, size, shape, and innervation of the vocal cords.

• Each of these are independently assorted, they do not come as a package.

• 3/10,000 chance that all determinants would be the same.• Depends on interaction of the components of vocal

apparatus; therefore, emergenic.

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Emergenesis of Geniuses: Carl Frederick Gauss

• Story of young Gauss: 1+2+3+4+5+6+7+8+9+10

• Was the product of the union of a bricklayer and a peasant.

• Intelligence is an emergenic trait.

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What does Pinker have to say?• “If people differ genetically in intelligence…could

we selectively breed for smarter people (no)…some traits such as genius are emergenic: they materialize only with certain combinations of genes and therefore do not breed true.”- Pinker

• Shockley’s Research: collecting sperm of Nobel Prize winners. One man replied, “If you want the sperm that produces Nobel Prize winners you should be contacting my father, a poor immigrant tailor. What have my sperm given the world? Two guitarists.”

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What other traits are emergenic?

• Aptitude, intelligence• Psychophysiological characteristics• Personality traits• Attitudes and interests• Brainwaves

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Do other emergenic traits correlate with Divorce?

Yes: Personality Traits

-42 % of the heritability for risk of divorce consisted of genetic factors affecting personality.

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Which specific personality traits?

• Traditionalism- following rules and authority, negatively correlated.

• Alienation- feeling mistreated and used, positively correlated.

• Dominance- being masterful and forceful, positively correlated.

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What do these correlations suggest?

• Personality and divorce risk are largely correlated as a result of common genetic influences.

• Genetic heritability of divorce risk found in twin research may actually be genetic heritability of specific personality factors.

• Interaction between personality factors of spouses can effect risk of divorce.

• But……

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....remember:

• Environmental influences: evidence from family studies show slight environmental influences.

• Cultural influences: Divorce not allowed in certain cultures.

-for example: one may have the genes for alcoholism but in cultures where there is no drinking, there are no alcoholics.

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Divorce is an example of an emergenic trait with many

influences

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Lykken says…

“Emergenic traits are like sewers; what you get out of them depends on what you put in them.”

-This Happy Breed

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Application to the Real World

• Emergenesis is just one more of nature’s many methods of ensuring variability in species.

• Humans remain creatures of genes, but we are not hostage to our ancestors.

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The End

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Key Points

• What do twin studies suggest for risk of divorce?

• What do these finding tell us about the nature of the genes influencing divorce?